Cardiac Electrophysiology I Flashcards
Resting Cardiac Muscle Cell
- biophysicists look at the cell from the inside with intracellular electrodes
- electrocardiologists look at the cell from the outside with extracellular electtrodes
Equilibrium potential
- voltage obtained for a given concentration gradient of a single ion at equilibrium across a semi-permeable membrane
- the equilibrium potential refers to a given single ion and is represented by the Nernst equilibrium equation
Gibbs Donnan equilibrium
- a special kind of equilibrium involving impermeable polyelectrolyte on one side of a membrane that is permeable to salts but impermeable to the polyelectrolyte
- exist across capillary membranes if albumin and other charged plasma proteins are in the blood but not in the interstitial fluid
- results in an unequal distribution of salts across the membrane and a slight membrane potential that has the same sign as the charge on the polyelectrolyte
Diffusion potentials
- when two or more ions are permeable to a membrane but the various ions have differing permeabilities
- calculated by the Goldman Hodgkin Katz equation
Epithelial membrane potentials
-are the differences of electrical potential that occur between two dilute solutions when the membrane itself is a layer of cells, such as occurs in the kidney and gastrointestinal systems
Equilibrium potential for specific ion
-Na+ +60 mV
-K+ -90 mV
-Ca++ +120mV
z= 1 for Na+ and K+, but is 2 for Ca++
-voltage gated Na+, K+, and Ca++ channel share a similar tetrameric molecular structure with each subunit of the tetramer consisting of six helical segments
Nernst Equilibrium Potential for K+
= -58 log ([Kin]/[Kex])
- raising Kex decreases outward K+ gradient and makes Ek less negative (more positive) which is depolarizing
- raising Kin increases outward K+ gradient and makes Ek more negative, which is hyperpolarizing
- at the low external K+, the membrane potential is more positive than predicted by the Nerst equation
- depolarization- means more positive
- hyperpolarization- means more negative
GHK Diffusion Potential
- Raising K(in) increases outward K+ gradient and makes Em more negative inside which is hyperpolarizing
- Raising K(ex) decreases outward K+ gradient and makes Em less negative (more positive) inside, which is depolarizing
- Raising Na(in) decreases inward Na+ gradient and makes Em less positive (more negative) inside, which is hyperpolarizing
- Raising N(ex) increasing inward Na+ gradient and makes Em more positive inside which is depolarizing
Biological Diversity of Cell Membrane Diffusion
- Neuron -70mV
- Skeletal Muscle -85mV
- Cardiac Muscle
- Atrial and Ventricular -80mV
- AV Node -65mV
- SA Node -55mV
- Smooth Muscle -55mV
- Secretory Cell -55mV
- Human Red Blood Cell -11mV
- the relative permeability of sodium to potassium determine cell resting potentials
Outward rectification
-the conductance of outward currents is greater than for inward currents, and the current voltage curve slopes upward nonlinearly
Inward rectification
- the conductance of inward currents is greater than for outward currents and the current/voltage plots slope downward nonlinearly
- K+ channel is inward rectifier
- when the cell is hyperpolarized at rest, the conductance of this channel is high, however upon depolarization during the upstroke of an action potential, the conductance of the iK1 channel becomes considerably less, allowing the inward sodium current to depolarize the membrane potential
- at normal resting potential, the inward rectifier channel iK1 mediates a positive efflux of potassium
Molecular structure of K channel
- 4 identical subunits
- the transmembrane domain forms the pore that allows the ions to cross the membrane
- the selectivity filter filters out ions other than K
Single File Electrodiffusion Through an Open Channel
-ions move through open channels by the process of electrodiffusion in single file, three K+ ions may simultaneously occupy a K channel but the ions do not pass each other in transit
Channel blockers
- like Ca++ channel blockers occlude the calcium channel at the extracellular side
- Calcium channel blockers can reduce the heart rate and contractility of the heart resulting in lower cardiac output and lower blood pressure
- ACE inhibitors blocking the conversion of angiotensin I to angiotensin II are also widely used to relax smooth muscle resulting in decreased TPR and lower blood pressure
How Membrane Voltage Changes when channels open
- whenever a cell becomes more permeable to an ion, the effect upon the cell’s membrane voltage is to bring it closer to the equilibrium potential of that particular ion
- when a cardiac muscle cell at Vm = -60 mV suddnely becomes very permeable to Na+, what will happen
- Ina increases greatly as Na+ flows into the cell
- the value of Vm readily moves toward +60 mV the value of Ena causing the depolarizing upstroke of the AP